Does e-mail forwarding count toward a customer's overall bandwidth limit for the month? i.e. if someone sends a customer a 10mb attachment, which is then forwarded to a third-party provider, does he get nailed for 10mb of bandwidth? Or maybe they get nailed for 20mb... 10mb of incoming bandwidth, and then 10mb of outgoing bandwidth? Or nothing at all?

AFAIK, they will be charged 10MB. As they should (though thinking about it they really ought to be charged 20MB since that is what it is costing you). Simplest test would be to try it out and see

A point to note: A 10MB attachement does not equal a 10MB mail. Presuming that it's a non-ASCII attachements, it will have to be encoded in BASE64 which usually bloats it to around twice the size. One reason why such large attachements are not appropriate for email.